Barrett's esophagus denotes the presence of specialized columnar epithelium (SCE) in distal esophagus. It develops in subjects with acid reflux disease as replacement for destroyed esophageal stratified squamous epithelium (ESSE). In 90%, it remains stable for life and irrespective of treatment since most - presumably because of its acid resistance - are (reflux) symptom-free. In one sense, then, Barrett's is a successful adaptation for esophageal protection. Barrett's, however, has a high cell turnover rate which establishes it as a premalignant lesion that increases esophageal cancer risk 40-fold over the general population. Though clinically important, little is known about the biology of SCE, and specifically about the mechanisms for acid resistance that are at the heart of its very existence. Therefore the goals of this proposal are: a) to define the preepithelial and epithelial mechanisms for SCE's survival under acidic stress: b) compare them to ESSE in Barrett's in order to test the hypothesis that SCE develops in GERD because of intrinsic mechanisms that make it more acid resistant than ESSE, and c) since alcohol and cigarette smoking increase the risk of malignancy in SCE, test the hypothesis that this risk reflects the ability of these agents to shorten cell survival I (which increases cell turnover) by impairing SCE's ability to protect itself under an acidic stress. The specific aims are to determine in SCE and ESSE in Barrett s: 1) the magnitude of the lumen-to-surface pH gradient and role of mucus and HCO3, 2) the permeability of the apical membrane-junctional complex to H+, 3) the total cell buffer capacity and role of carbonic anhydrase, 4) the nature, location and activity of transporters for pHi regulation, 5) the effect of alcohol and smoking on cell survival under an acid stress, and to: 6) compare the results to ESSE in those without esophageal disease. Studies are performed using esophageal biopsies for cell isolation, culture and mounting in mini-Ussing chambers. Techniques include: pH icroelectrodes, immunohistochemistry, impedance analysis and fluorescence microscopy. Achievement of the proposed goals will yield information of fundamental importance to understanding the pathogenesis and malignant potential of SCE in reflux subjects and provide insights into possible novel strategies for its prevention, stabilization or selective eradication for reduction of cancer risk.